1. Field of the Invention
This invention relates to methods of making new and improved diamond, "diamond-like carbon" (a-C) and "diamond-like hydrocarbon" (a-C:H) coatings bonded to substrates by using intermediate bonding layers engineered to reduce the residual stress in the diamond coatings, and to articles of manufacture made using such methods. The field of the invention also includes computer control systems for application of the diamond film via chemical vapor deposition or other techniques in such a way as to minimize the dynamic mismatch in strain during cooling of the coated product.
2. Background of the Invention
Diamond, diamond-like carbon and diamond-like hydrocarbon coatings have been employed both to provide hard faces on engineered materials and as abrasive coatings on articles made from such materials. Typically such diamond films and/or particles are applied using some form of chemical vapor deposition (CVD) process. Such processes generally use thermal decomposition of a mixture of hydrogen and carbon compounds, preferably hydrocarbons, into diamond generating carbon atoms preferentially from the gas phase activated in such a way as to avoid substantially the deposition of graphitic carbon. The specific types of carbon compounds useful for CVD include C1-C4 saturated hydrocarbons such as methane, ethane, propane and butane; C1-C4 unsaturated hydrocarbons such as acetylene, ethylene, propylene and butylene; gases containing C and O such as carbon monoxide and carbon dioxide; aromatic compounds such as benzene, toluene, xylene, and the like; and organic compounds containing C, H, and at least one of oxygen and/or nitrogen such as methanol, ethanol, propanol, dimethyl ether, diethyl ether, methylamine, ethylamine, acetone, and similar materials (see U.S. Pat. No. 4,816,286). The molar concentration of carbon compounds in the hydrogen gas can vary from about 0.1% to about 5%, preferably from about 0.2% to 3%, and more preferably from about 0.5% to 2%. The resulting diamond film in such a deposition method is in the form of adherent individual crystallites or a layer-like agglomerates of crystallites substantially free from intercrystalline adhesion binder.
Such CVD processes are known to those skilled in the art, and ordinarily use some form of energy (for example, microwave radiation, as in U.S. Pat. No. 4,859,493 and in U.S. Pat. No. 4,434,188) to pyrolyze hydrocarbon gases such as methane at concentrations of about 1% to 2% in a low pressure (about 10 torr) hydrogen atmosphere, causing deposition of diamond or "diamond-like carbon" (a-C) or "diamond-like hydrocarbon" (a-C:H) particles or film on a nearby substrate. (Diamond and "diamond-like carbon" (a-C) coatings have an atomic hydrogen fraction of zero; for "diamond-like hydrocarbon" (a-C:H) coatings that fraction ranges from about 0.15 to about 0.6. Diamond coatings have atom number densities around 0.29 gram-atoms per cubic centimeter; "diamond-like carbon" (a-C) and "diamond-like hydrocarbon" (a-C:H) materials are characterized by atom number densities above 0.19 gram-atoms per cc.) It is also known to assist the CVD process using a variety of techniques including (1) pyrolysis by a hot tungsten filament intended to generate atomic hydrogen near the substrate (HFCVD); (2) supplying electrons by negatively biasing the filament as in electron-assisted chemical vapor deposition (EACVD); (3) creating a plasma using microwave energy or RF energy (PACVD; see U.S. Pat. Nos. 4,504,519 and 5,382,293); (4) using an argon ion beam to decompose the hydrocarbon feedstock, as in U.S. Pat. No. 4,490,229 and (5) using direct-current electrical discharge methods. See, generally, John C. Angus and Cliff C. Hayman, "Low-Pressure, Metastable Growth of Diamond and `Diamondlike` Phases," Science, Aug. 19, 1988, at p. 913. The disclosures of the U.S. patent references cited above are incorporated by reference herein.
The ion beam deposition method typically involves producing carbon ions by heating a filament and accelerating carbon ions to selected energies for deposit on a substrate in a high vacuum environment. Ion beam systems use differential pumping and mass separation techniques to reduce the level of impurities in the carbon ion flow to the growing film.
The chemical vapor deposition and plasma enhanced chemical vapor deposition methods are similar in operation. Both methods use the dissociation of organic vapors (such as CH.sub.3 OH, C.sub.2 H.sub.2, and CH.sub.3 OHCH.sub.3) to produce both carbon ions and neutral atoms of carbon for deposit on a substrate. Plasma enhanced methods are described in U.S. Pat. Nos. 5,382,293 and No. 5,403,399.
Non-hydrogenated diamond-like carbon (a-C) films can be applied using a variety of techniques, which include magnetron sputtering, electron beam physical vapor deposition (EBPVD), laser photo-ablation, mass-filtered carbon ion beam deposition and cathodic arc plasma deposition, as described in U.S. Pat. No. 5,401,543 (incorporated by reference herein).
Sputtering deposition usually includes two ion sources, one for sputtering carbon from a graphite source onto a substrate, and another ion source for breaking the unwanted graphite bonds in the growing film. In the typical sputtering method, an argon ion sputtering gun sputters pure carbon atoms off of a graphite target within a vacuum chamber, and the carbon atoms are condensed onto a substrate. Simultaneously, another argon ion source bombards the substrate to enhance the breakdown of the graphite bonding in favor of a diamond-like sp.sup.3 tetrahedral bond in the growing carbon film.
It is also known to apply polycrystalline diamond layers using sintering at simultaneous high pressures (50 kbar) and temperatures (1300.degree. C.) to create conditions under which the diamond phase is thermodynamically stable, as in U.S. Pat. No. 5,370,195. And liquid-phase diffusion metallizing techniques also have been suggested for bonding diamond to certain types of substrates, as in U.S. Pat. No. 5,392,982.
Synthetic diamond-coated articles have found a wide variety of uses. U.S. Pat. No. 4,960,643, for example, discloses articles coated with synthetic diamond particles of controlled size, to which an overlying film, for example of chromium, has been applied to lubricate the diamond layer and to help resist scratching and wear. Other patents disclose various diamond-coated articles of manufacture, including bearings (U.S. Pat. No. 5,284,394); fasteners (U.S. Pat. No. 5,096,352); engine parts (U.S. Pat. Nos. 5,132,587 and 4,974,498) and the like.
The usefulness of diamond-coated engineered materials, and especially those made by CVD techniques, has been limited, however, by the large residual stress which remains in the finished composite products after coating is complete and the coated article has cooled. The stress arises from the very large differences in coefficients of thermal expansion (CTE) between the diamond coating (which have very low CTE's) and the substrates to which it is desired to bond it (often a metal having a much higher CTE). The substrate contracts during cooling more than the diamond film, leaving the diamond film in a permanent state of compressive stress which promotes spalling and cracking. The effect is aggravated by the high elastic modulus (low compressibility) of diamond compared to that of the substrate.
The rate of cooling of the substrate and the diamond film also affects the amount of stress, since diamond has a high thermal conductivity (promoting faster cooling) than metal substrates. Thus, the stress created during cooling may even exceed the static residual stress that remains when both the substrate and the film have reached the same final temperature.
Prior attempts to address the problem of fragility and crack propagation caused by residual stress have included providing soft, metallic braze layers, as in U.S. Pat. No. 4,968,326, sometimes molybdenum-based or carbide-based, as in U.S. Pat. No. 4,776,862. It has also been suggested to supply an intermediate braze layer in the form of multiple elements such as discs with dissimilar centers and edges between the metal and the diamond being bonded together. See U.S. Pat. No. 5,392,982. Such complex, multi-part intermediate layers are unsuited to situations in which uniform properties over a comparatively large coating area are desired and would also be difficult to adapt to CVD diamond film application techniques. Similarly, techniques that require drilling holes through the diamond layer to mechanically assist in bonding, as in U.S. Pat. No. 5,239,746, are excessively complex and costly for any high-volume applications. Additionally, the rough edges of such drill-through holes become crack initiation sites for spalling failures.
Others have suggested simultaneously co-depositing a silicon carbide onto a molybdenum substrate, along with the diamond. See U.S. Pat. No. 5,190,823. Such a technique, however, requires weeks of deposition time and is impractical for commercial purposes. And, it fails to address the problem of ameliorating the differences in CTE between the diamond and the substrate: the SiC has a CTE much closer to that of diamond than to those of steel or aluminum alloys, as does Si.sub.3 N.sub.4, which has also been suggested as a base for sinter coating. See U.S. Pat. No. 5,137,398. Mixing SiC or Si.sub.3 N.sub.4 with diamond in a single, mixed coating does still leaves a likelihood of unacceptably high residual compressive stress. And, along with the low CTE, such materials lack the ductility and other desirable properties of metal substrates.
Still other investigators have suggested depositing multiple-layer polycrystalline diamond films by HFCVD, with cooling periods between layers of diamond film. See U.S. Pat. No. 5,124,179. This technique, however, also requires prolonged deposition times.